The invention relates to a method for communication by radio between a device and radio stations of a radio communications system. Furthermore the invention relates to a device for communication by radio with radio stations of a radio communications system.
In radio communications systems information (for example speech, picture information, video information, SMS (Short Message Service) MMS (Multimedia Messaging Service) or other data) is transmitted with the aid of electromagnetic waves over a radio interface between transmitting and receiving station. The radio stations can in this case, depending on the actual embodiment of the radio communications system, be various types of subscriber-side radio stations, radio access points or base stations. The electromagnetic waves in such systems are radiated using carrier frequencies which lie within the frequency range provided for the relevant system.
Radio communications systems are often embodied as cellular systems, e.g. in accordance with the GSM (Global System for Mobile Communication) or UMTS (Universal Mobile Telecommunications system) standard, with a network infrastructure includingbase stations, devices for checking and control of the base stations and further network-side devices. Frequencies at 900, 1800 and 1900 MHz are used for the cellular GSM mobile radio system.
As well as these cellular, hierarchical radio networks organized on a wide-area (supralocal) basis, there are also Wireless Local Area Networks (WLANs) with a radio coverage area that as a rule is far more limited. The cells covered by the radio access points (AP) of the WLANs, with a radius of up to a few hundred meters, are small by comparison with usual mobile radio cells. Examples of different standards for WLANs are HiperLAN, DECT, IEEE 802.11, Bluetooth and WATM.
Generally the non-licensed frequency range around 2.4 GHz is used for WLANs. Data transmission rates range up to around 11 Mbit/s. Future WLANs can be operated in the 5 GHz range and will achieve data rates of over 50 Mbit/s. This provides subscribers of the WLANs with data rates which are significantly higher than those offered by the third mobile radio generation (such as UMTS for example). This means that for the transmission of large volumes of data, especially in connection with Internet accesses, access to WLANs for high-bit-rate connections is advantageous.
A connection to other communications systems, for example to Broadband Data Networks (BDN) can thus be made via the WLAN radio access points. To this end the radio stations of the WLAN communicate either directly with a radio access point or with more remote radio stations via other radio stations which forward the information between the radio station and the radio access point over a routing between the radio station and the radio access point.
In an ad-hoc mode of a radio communications system radio stations can communicate with each other via one or more hops (hop, multihop) without the intermediate connection of switching devices such as base stations or radio access points for example. If for example a radio station intends to transmit messages to another radio station outside the radio coverage area, as is often the case in an ad-hoc mode for example, a route must then first be determined between the transmitting and the receiving radio station. A route passes through one or more radio stations which forward messages for a transmission between transmitting and receiving radio station. The radio stations of the route are thus directly adjacent radio stations which can communicate with each other directly via radio.
Document WO 01/28170 A2 describes an ad-hoc computer network in which packets are sent. The nodes of the network send “scheduling packets” with information about other nodes which are located up to two hops away from the relevant node. The “scheduling packets” also contain information about occupied and free channels.
A routing can for example be determined by radio stations using a variety of methods for decentralized routing determination. A plurality of signaling messages are generally sent in this case, so that a significant strain can be imposed on the scarce radio resources through the determination of the routing. As an alternative a routing can also be determined by a centrally supported method for routing determination which includes a device which knows about adjacent relationships between radio stations or about the current network topology. After the device has notified the radio stations about the routing which it has determined, the radio stations can then transmit messages over the routing, in which case, for each message forwarded, the relevant forwarding radio station has to have access to the radio resources. In accordance with the IEEE 802.11 standard this type of access is undertaken for example using the CSMA (Carrier Sense Multiple Access) method. The disadvantage of this however is that the complete transmission of a message over the routing can be delayed because of unsuccessful access to the radio resources or because of the fact that an access is not possible because the radio resources have been reserved by other radio stations, so that the routing becomes outdated in the meantime and complete message transmission is no longer possible via this routing Furthermore, where there is uncoordinated access by the transmitting radio station and the radio stations of the routing to the radio resources, it is possible for a radio station to receive so many messages for forwarding that its memory becomes overloaded, resulting in messages being lost or having to be discarded.